On William McNeill's Plagues and People.
The bovine-to-human link in the BSE affair will, if confirmed, underscore a timeless truth. The world is full of microbes manoeuvring for supplies of nutrients, energy and molecular building blocks. Via the relentless probing processes of natural selection, the bacteria, viruses and prions occasionally discover a new bonanza. The right mutant microbe in the right place can found a micro-dynasty.
Two decades ago, the American historian William McNeill wrote a wonderful book, Plagues and Peoples. Traversing the sweep of human cultural transformation over the ages, he described a world of restless parasitism in which the human species has always been just one player. Meanwhile, most western epidemiologists were busy addressing the modern disease agenda. Infectious diseases were passe; the Big Deal was heart disease, cancer, diabetes, asthma and so on.
Our confident assumptions about the "epidemiological transition", in which old infectious diseases have been traded for new non-infectious diseases, were misplaced. Yes, we noted a few unsettling signs in the 1970s - Legionnaire's disease, Lyme disease, "swine-flu" - but mainstream epidemiologists hardly noticed them. Meanwhile, new haemorrhagic fever viruses were surfacing in Third World rural populations, as forests and ecosystems were disturbed. Drug-resistant bacteria and malaria were becoming troublesome. But this all seemed peripheral to the main plot.
Then, in the 1980s, HIV changed the plot. Infectious disease had to be taken seriously. Resurgent cholera and tuberculosis and new shock-horror stories out of darkest Africa followed in the 1990s.
Beneath the hyperbole is a substantial message. As human ecology changes, so the prospects for infectious diseases change. We are inextricably bound up with the natural world, a world of competition and symbiosis between organisms great and small.
When I read Plagues and Peoples in 1991 these ideas became clear, and enthralling. I was then exploring how environmental changes might affect human health, especially by perturbing infectious diseases such as malaria. McNeill's lucid analysis was revelatory. Here was an account of the evolving infectious disease profiles of human populations as they changed from nomadism to agrarianism to industrial urbanisation. Here was a cautionary tale about the consequences of intensive production of livestock. Tuberculosis and smallpox came as mutant microbes from ancient infections in cattle, measles from dogs, influenza from chickens, leprosy from water buffalo, the common cold from horses, and so on. When humans congregate and live close to domesticated animals, the microbes, true to the evolutionary imperative to explore new ecological niches, jump species.
The book surveys grand historical vistas: the merging of the disease pools of great civilisations. Two thousand years ago, the Roman Empire and Han dynasty, the poles of Eurasia, made contact. Epidemiological catastrophe occurred; virulent new plagues ravaged both populations. The eventual grand pooling resulted in an uneasy Eurasian equilibrium.
Transatlantic contact followed. The native populations of the Americas were devastated by European infections - and may have sent several back in return. Meanwhile, European armies began trading syphilis, typhus and other infections.
The best historians have something prophetic to say. I wish I had discovered McNeill's book a little earlier.
Tony McMichael is professor of epidemiology, London School of Hygiene and Tropical Medicine.